Ch. 12 Introduction

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Ch. 12 Contents

General
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------------------Online--------------------------------------- Ch. 12: The Cell Cycle: cell division by binary fission or mitosis

Introduction

The ability to reproduce is one important characteristic of living protoplasm. This chapter looks at the mechanics of cell reproduction. These mechanics are the basis of understanding:

reproduction of single celled bacteria or protists
growth of a single fertilized egg into the trillions of cells making up the adult
repair of wounds and damaged tissues
growth of cancer tumors

Cells reproduce by cell division, a process involving:

duplication of DNA
separation of DNA to opposite ends of the cell
separation of cytosol into two cells

Cell division produces two "daughter cells" that are genetically identical copies of the parent cell, or clones of the parent.

What would happen if a cell failed to duplicate its DNA prior to dividing?

a. cells can make more DNA anytime, so it would not be a problem
b. the DNA would be reduced by one half each division
c. the DNA would be reduced by one quarter each division

answer

You need to understand a few basics about the nature of the DNA molecules that make up your chromosomes. Click here.

Prokaryotic cells (bacteria) divide by a simpler process than eukaryotic cells, this process is called binary fission. The "Cells Alive" Web site has a good animation showing one cell dividing into two, two into four, etc. (http://www.cellsalive.com/qtmovs/ecoli_mov.htm)

Eukaryotic cells divide by a process involving mitosis. The "Biology Project" Web site at the University of Arizona (link below) has a good animation showing the interphase, the normal nondividing phase of a cell's life cycle, and stages of mitosis:

prophase, the coiling of chromatin, strands of DNA, into tight condensed chromosomes
prometaphase, the nuclear membrane dissolves and microtubules attach to chromosomes
metaphase, the chromosomes align along the equator of the cell
anaphase, the chromosomes (chromatids) are pulled apart by shortening microtubules
telophase, the uncoiling of chromosomes and reformation of nuclear membrane and separation into two daughter cells, a process called cytokinesis.

University of Arizona link: http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells3.html: Click on "Mitosis Animation" link on the bottom left of the page.

A normal cell spends most of its life just performing its job, e.g. muscle cell, red blood cell, nerve cell, etc. Some cells such as skin, epithelial lining of mucus membranes, and bone marrow cells divide a lot. Other cells such as nerve cells and muscle cells divide very little once the fetus is fully formed. This is why the destruction of cells of the mucus membranes, e.g. flu virus, causes only temporary damage to these tissues. This reproductive ability is also why minor wounds to the skin are easily repaired. However, injury to nerve tissue, or destruction of nerve tissue by a virus, e.g. polio virus, causes permanent damage to the tissue.

Cells are triggered to divide by physical and chemical signals. These signals trigger duplication of the nuclear DNA and then its separation in mitosis.

Cell Cycle

Organisms have a life cycle: they are born, grow old and die, and most reproduce sexually one or more times during their life. An individual cell in our bodies has a different kind of life cycle, they do not grow old and die but reproduce over and over again producing a line of offspring called a "cell line," or cell lineage. In most mammal tissues, cell lines can go for about 50 generations before the cells appear to grow old and the organism dies. In some single cell creatures like an amoebae, the cell lines are immortal. To see a drawing illustrating one cell from a single celled creature (e.g. amoeba) and one from a multicellular creature (e.g. human) that have each completed three cycles of the cell cycle producing a cell line containing 8 cells, click here.

One cycle in the life of a eukaryotic cell is illustrated in the figure below. The blue circle is like the face of a clock, measuring the passage of time in the cell's life cycle. Notice that the cycle is divided into interphase and the phases of mitosis: prophase, (prometaphase omitted), etc.

Remember back to when you were a fertilized egg cell in your mother's womb. One trip around the blue clock will tract the events taking you from one cell to two cells. You start as a single cell in interphase at around 7 o'clock on the blue clock face. At what time do you complete G1,S and G2 of interphase and start mitosis? ___ o'clock.

That interphase you went through is divided into three phases: G1, S and G2. If the cycle time was 12 hours, how long did G1 last? approx. ___ hours. How long did S last? ___ hours. How long did G2 last? ___ hours.

Now lets look at the yellow cell with a gray nucleus in G1 of Interphase, 7-10 o'clock. Assume this is you as a fertilized egg. The gray is your nucleus, surrounded by yellow cytosol. Over the next few hours of interphase you are going to duplicate your DNA strands and get ready to divide them during mitosis. And for simplicity, I have drawn you with just one pair of DNA strands. The red strand came from your mother and the blue strand from your father. You really have 23 pairs, but this figure is complicated enough as it is.

As you sit there in G1, you are absorbing sugar through your membrane, your are making lots of mRNA and translating it into proteins to carry out cellular respiration and making lots of ATP. You have a lot of work to do (growing into a baby and then adult) and will need lots of ready chemical energy.

Then you receive a chemical signal. Perhaps it was one of the proteins you made; perhaps it was a chemical you absorbed from your mother's uterus. This chemical signal tells you it is time to stop G1 activity and start duplicating your DNA. You start the S phase, and when you have made an exact copy of each DNA strand, the S phase is complete. Now you are in the G2 phase, and you get busy making all the enzymes and ATP you will need to do the work of separating you DNA during mitosis. When you have all the chemicals you are going to need, you start mitosis.

At about what time on the clock face do you start mitosis? ___ o'clock.

The first phase of mitosis, prophase, involves coiling up your DNA strand into a chromosome. Notice the duplicated strands of DNA are attached to each other, like a pair of long silk stockings tacked together with a needle or pin. When you have finished coiling up your strands of DNA into chromosomes, you line up the chromosomes, all 23 pairs, along the equator of the cell (metaphase = alignment phase). Then you move the chromosomes apart (anaphase = movement phase). When your chromosomes are at opposite ends of your cell, you begin to uncoil them, reforming them inside new nuclear membranes (telophase). Then you cleave the cell into two cells. Mitosis is complete and you are back in a new G1 phase. Now you are two cells.

You repeat this process about 30 times ( 1 cell to 2 cells, 2 to 4, 4 to 8, 8 to 16, etc.), and after nine months you have grown into several trillion cells. Then you send you mother a chemical signal saying, "Its time!" And contractions start.

In the figure, around the outside of the cell cycle, there are small square photographs of actual cells going through the various phases. These are onion cells. Look at the onion cell at nine o'clock. The nucleus is gray, and the nucleolus is visible as a darker spot. The DNA threads are not coiled up yet and can't be seen, except as gray chromatin.

Track the onion cell through all phases of the cell cycle.

On the exam you will be shown onion cells and asked what phase they are in.

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